Selective carburization of metals



Patented Oct. 18, 1949 2,485,176 SELECTIVE CABBURIZATION F METALS Frederick 11). Waterfall, Warley, Birmingham,

England, assignor to Imperial Chemical Industries Limited, a corporation of Great Britain N0 Drawing. Application August 19, 1946, Serial No. 691,669. In Great Britain August 27, 1945 6 Claims.

This invention relates to improvements in the heat treatment of metals and to compositions for use in such processes.

It is frequently desired to subject ferrous articles to a case-hardening process, that is, a process in which the surface layers of the articles are hardened. One suitable method for carry ing out this is to immerse the articles in a fused salt bath containing a cyanide, for example a bath consisting of a mixture of sodium cyanide with sodium carbonate, or a mixture of sodium cyanide with one or more alkali metal chlorides and/ or alkaline earth metal chlorides. It is sometimes desirable to harden only a part of the surface and in such processes this is usually done by coating that part of the surface which it is desired not to harden with copper, for example by electro-deposition. This part of the surface is then said to be stopped-off and the copper is said to be a stopping-off agent. It has been proposed to protect parts not to be hardened by a metal coating comprisin at least one metal melting above the hardening temperature and one metal melting below that temperature. The metals could be alloyed or applied successively or in admixture, for example by spraying. It has further been proposed to use as a stopping-off agent a liquid vehicle adapted to dry to an adherent film, in which vehicle is suspended a mixture of two powdered metals, one melting below the carburising temperature and the other above that temperature and being present in an amount between 4 and 50 times that of the former metal. In supplying such a coating to a metal surface it is necessary for the best results that the coating metal should present a continuous barrier to" the fused salt bath throughout the period of immersion of the article in the bath so that local hard spots may not occur in the area which is supposed to remain soft. The adequacy with which this can be done depends in part on the readiness with which the composition can be applied as a free-flowing coating of gOOd covering power. Such coatings containing metallic powders of comparatively coarse particle size may have rather better covering power than those containing very fine powders such as the fine flake bronzes used in decorative paints, probably on account of the more rounded shape of the coarser particles. These rather coarse metal powders are however difficult to use effectively in stopping-01f paints since the resulting compositions are not usually sulficiently free-flowing and are not therefore easily applied to the metal surface.

It is now found that this difiiculty can be overcome by adding to the metal powder a finely-divided lead oxide, such as the red lead customarily used in paints.

According to thepresent invention, therefore,

2 a stopping-off composition for use in selective carburisation having both free-flowing properties and good covering power comprises a solution in a volatile solvent of a lacquer which dries to a hard film having suspended therein a metal powder formedof copper mixed or alloyed with up to of tin together with a minor proportion of a lead oxide or compound of lead capable of decomposing to give a lead oxide at the temperature of the carburisation bath.

By including such a lead compound a composicompound. Moreover, the protective value of the composition is substantially improved, so that the non-coated portions of the ferrous article can other solvents which may be used include such compounds as n-butyl alcohol, n-propyl alcohol or cyclohexanol, or mixtures of these with aromatic hydrocarbons such as toluene or xylene. The lacquer is conveniently of an air dryin type but other forms of lacquer, such as for instance those which require stoving to harden, could also be used, since the lacquer functions chiefly as a carrier for the metallic pigment.

The pigment may be composed of a bronze alloy powder containing between 5% and 15% and preferably between 5% and 10% of tin. Instead of using a bronze powder a mixture of powdered copper with powdered tin may be used, the mixture containing 5% to 15% of tin. Proportions of tin above 15% are not desirable since the melting point of the alloy or mixture thus becomes too low. The said alloy or mixture must be capable of covering the metal surface satisfactorily at the carburisation temperature, but must not be fluid enough to flow on the part durin treatment so as to spread over the surface on to the portions which are required to be hard. The particles of metal powder should be at least fine enough to pass through a 1'70-mesh B. S. S. sieve, although it is preferable to use a still finer powder. The material used in the examples hereinafter described was of 300-mesh particle size and a satisfactory stopping-off composition was prepared from it, but powders of finer mesh, say 400-500, can also-be used.

The lead compound should'be present in the pigment in minor proportion, that is it should constitute less than 50% by weight of the total pigment. It may, however, :suitablybezpresent in any amount from 5% to 20%, the balance consisting of the metal powderor;:.powders. It'is preferred to use a pigment containing between and of the lead compound,'the remaining 90-80% comprising a copperetinrmixtureaor alloy composed of 86-68% by *weightpf 'copper and 4-12% by weight of tin. The said compound should be a lead oxide-or a compound of lead which decomposes to give a finely-divided lead oxide at the temperature of the carburisation bath. Red lead'is found-to-give very satisfactory results, but other-lead oxides can be'used or compounds such as for instance-the hydroxide or basic carbonate (white lead) which decompose at high temperatures to give the oxide.

The-proportion of vehicleto pigment is not criticalproviding thereis sufiicient of the former to bindall of the latterafter the vehicle has dried; in general the vehicle may form'between 40% and 60% by'volume of the composition. Within-these limits this figure may be determined by one skilled in theaart by taking into consideration, for examplegthe constitution of the pigment and its general physical state and the thickness or bo'dy" of the vehicle. It-is found that approximately-equahparts by 'volume of pigment and vehicle-give a paint of suitable consistency which iseasily applied to the metal parts and which allows the pigmentto be evenly distributed thereon. The vehicle and pigment should preferably be mixed when the-composition is required for use, but they may also be previously mixed and stored in the mixed condition. Smallamounts of substances known .to .actas plasticisers and driers may also be included in vthe composition if desired.

The stopping-oil agent maybe usedin an analogous methodto that which has ben used hitherto. Thus after preparingthe ferrous article by grinding, degreasing or other treatment, those portions of the article 'which it is notrequired to case-harden are coatedwith the composition by brushing, dipping, orspraying; and the latter is allowed to dry in the air. By choice of a suitable solvent the composition dries-quickly and after-a period of only'two hours coated articles maybe treated in thecarburising-bath, no loss of efficiencybeing observed with use of so brief a drying time. His, ahowever, desirable to allow a longer time for drying in cold or humid weather, and the articles are then advantageously left'to stand overnight. They are immersed in the casehardening bath until the necessary carburisation of the exposed surfaces has been eifected and are .then removed from the bath. The time of exposure to the action of the bath may vary from 'one hour to as much asssix hours, according to are-ssuitableiforrcarrying out the case-hardening process. Baths of higher cyanide content up to say 40% to 50% NaCN may also be used for carburisation, but are ingeneral more prone to penetrate =the stopping-elf 'compound than those of lowcyanide content. The composition according to the invention gives protection for at least two hours at 850"950 C. in the baths of high cyanide content-and in the case of baths of lower cyanide content complete protection is obtained for from four to six hours.

After removal-from the'bath, the articles are cooled inairyoil or'water, a'ccording'to the requirements-in relation to the parts being treated.

The "following examples illustrate but do not limit the invention, all parts being by weight:

Example. 1

88 parts of a tin-bronze powder containing 10% oftin and-capableof,passingthrough a 300- mesh -B. S. Sasieve .were intimately mixed with 12 parts-of finely-dividedred-lead. A vehicle was prepared by dissolving -a phenol-formaldehyde resin-in-n-butyl alcohol, and equal volumes of the bronze-red.lead mixture and the vehicle were mixed until the composition had a smooth and even consistency suitable for .use as a paint. Specimens .of mild steel weredegreased and polished'to removeall ldirt'and scale .and were then partially .coated with the stopping-0E composition by means of a brush andallowed to dry in the-air overnight. The samples were immersed in the molten carburising bath atatemperature of 950 C. for periods of .1, 2, 4 and'6 hours respectively,.one set being treatedin a'bath of high cyanidecontent and the other in a bath'containingup to 20% cyanide andreferred to hereafter as .the low-cyanide bath. After immersion for theprescribedtimethe samples were withdrawn and cooled .slowly in air. Microsections were taken fromthe paintedand unpainted zones in order to .determinethe depth of, penetration. The results, shownin the following table, show that the composition islcompletely efiective as a stopping-.oifmediumat 950C. for 2 hours with the high cyanide bath and for .4hours with the low cyanide .bath. Both .the .total and eutectoid case depths lfor. painted and unpainted sections are iven in the table.

Oase Depths Low Cyanide Bath High Cyanide Bath Immersion time, Hours Painted Unpainte'd Painted Unpainted Total Eutectoid Total Eutectoid Total Euteotoid Total Eutcctoid Inches Inches Inches Inches Inches Inches Inches Inches 1 .02 .012 02 007 2 i036 018 004 032 .011 .052 .026 .014 ..046 .015 6 065 O32 032, 010 058 020 Example 2 A stopping-oil composition was prepared in the same way as in Example 1 using 80 parts of a 10% tin-bronze powder of 300-mesh and 20 parts of red lead, the vehicle being the same as in the previous example. The composition was brushed on to cleaned and degreased-mild steel samples and the latter were treated for 1, 2, 4, and 6 hours in baths of low and high cyanide content. The results are shown in the following table.

1. A stopping-off composition for use in the selective carburisation of metal articles comprising a liquid vehicle adapted to dry to an adherent hard film having suspended therein as pigment a metal powder consisting of copper and from 5% to 15% of tin together with a lead compound selected from the group consisting of lead oxide, lead hydroxide and basic lead carbonate in an amount equal to from 5% to 20% by weight of the total pigment.

2. A stopping-off composition according to claim 1 in which the lead compound is red lead.

3. A stopping-off composition for use in the selective carburisation of metal articles comprising a liquid vehicle adapted to dry to an adherent hard film having suspended therein as pigment 1,366,305

a metal powder consisting of a tin-bronze alloy of to 15% tin the particles of which are at least fine enough to pass through a 300-mesh B. S. S. sieve together with a lead compound selected from the group consisting of lead oxide, lead hydroxide and basic lead carbonate in an amount equal to from 5% to 20% .by weight of the total pigment.

4. A stopping-01f composition according to claim 3 in which the lead compound is red lead.

5. In the selective carburization of metal articles, the step of coating the part not required to be carburized with a composition comprising a liquid vehicle adapted to dry to an adherent hard film having suspended therein, in addition to 86-68% by weight of copper and 4-12% 'by weight of tin, -20% by weight of a lead compound selected from the group consisting of lead oxide, lead hydroxide and basic lead carbonate.

6. In the selective carburization of metal articles, the step of coating the part not required to be carburized with a composition comprising a solution of a phenol-formaldehyde resin in a volatile solvent having suspended therein a pigment comprising a powdered tin-bronze alloy of 5% to tin, the particles of which are at least fine enough to pass through a 300-mesh B. S. S. sieve together with red lead in an amount equal to 5% to by weight of the total pigment.

FREDERICK D. WA'I'ERFALL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Jackson Sept. 27, 1870 Whyte Jan. 18, 1921 Number 

